diag_read uses dma_alloc_coherent to allocate memory requested by the
caller. If this memory requested is larger, more than DIAG_TRANSFER_LIMIT
(2K), then it is likely that we may not get the requested memory and we
would fail.

To solve this, request dma_alloc_coherent for only DIAG_TRANSFER_LIMIT, and
reuse this buffer multiple times as needed to copy the data requested in
smaller chunks of size not more than DIAG_TRANSFER_LIMIT. Previously we
were reading into the caller's only after getting the complete requested
data.

Fixes: 68c03249 ('ath10k: convert pci_alloc_consistent() to dma_alloc_coherent()')
Signed-off-by: Ashok Raj Nagarajan <arnagara@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>